Only a handful of materials provide the highly useful combination of electrical conduction and optical transparency. The earliest used materials were thin films of gold, silver and platinum used as electrodes on photoelectric cells. Wide band-gap semiconductor transparent electrodes including tin oxide coatings came into use during World War II (Nesa glass), and indium oxide coatings became commercially available in the 1970's. See G. Haacke, "Transparent Conducting Coatings", 7 Ann. Rev. Mater. Sci. 73-93 (1977).
Transparent conductive materials are important components in visual communications devices and useful in a variety of other applications. In modem liquid crystal displays, for example, arrays of transparent electrodes define the display segments or even pixels to be activated. Moreover transparent conductors can be used as antistatic coatings on instrument panels, heating elements on aircraft windows, and electrodes on solar cells. Such materials are also useful as heat retaining and ultraviolet light-rejecting coatings on windows.
One difficulty with presently available transparent conductive materials is their narrow transmission spectrum. They tend to absorb blue light in the visible spectrum and some tend even to absorb red light. For example, indium tin oxide, widely used because of its high conductivity, tends to absorb blue light and, as a consequence, appears yellow or green. Such absorption requires higher power levels, limits choice in coloration, and presents an impediment to the development of full color flat panel displays. Presently known transparent conducting materials are absorbing in the infrared portion of the electromagnetic spectrum. Conducting materials that are transparent in the infrared can also to be useful for devices emitting at these wavelengths, for example, surface-emitting lasers operating at 980 nm, 1.3 .mu.m or 1.5 .mu.m.